1. Field of the Invention
The present invention relates to storage-stable postcrosslinkable compounds having at least two organopolysiloxane units and at least one methylol group, and to their preparation and use.
2. Description of the Related Art
Silicones and silicone-containing formulations and composites are known and are widely used in the form of films, coatings and overcoatings for modifying and enhancing a very wide range of materials of construction and fibers. Silicones and silicone-containing formulations have a performance spectrum that makes them in many respects superior to purely organic films, coatings and overcoatings. The use of silicone products thus leads to a substantial improvement in otherwise unobtainable but generally desirable properties such as for example flow behavior, gas permeability, abrasion resistance, hydrophobicity, smoothness, haptics or luster on the part of the treated substrate.
An immense problem with all coatings, but in particular with silicone coatings and their limited chemistry, is the often poor permanence on the particular treated substrate. The consequence of this poor permanence is that the coating is simple to remove mechanically, for example by rubbing or scuffing, or may become detached again from the substrate as a result of chemical stress, for example contact with various solvents and/or exposure to certain pH environments (as occur in washing operations for example).
One approach to solving the problem of poor permanence consists in crosslinking the individual silicone polymer chains with the substrate to be treated as well as with each other, and so increasing the mechanical and chemical resistance and hence permanence of the overall system. The crosslinking between the chains and the bonding to the substrate may be effected not only via noncovalent interactions but also via covalent bonds.
Hydrogen bonds in particular have become established among the noncovalent interactions. Hydrogen bonds, formed for example in the form of urethane or urea groupings within the group of thermoplastic silicone elastomers, combine to ensure an increased network density and also, by interacting with substrate groups which likewise form hydrogen bonds (for example hydroxy units in the case of cellulose surfaces), a certain degree of fixation. The preparation and use of such thermoplastic silicone elastomers is described at length in the publications EP 0 606 532 A1 and EP 0 342 826 A2 among others.
A different noncovalent mechanism of crosslinking involves acid-base interactions between Lewis-basic/Lewis-acidic groups of the silicone polymer with Lewis-acidic/Lewis-basic groups of the substrate or of the polymer. Examples thereof are amino-functional silicone oils which, as will be known, have a positive influence on the hydrophobicity and softness of textiles in particular and, by virtue of their Lewis-basic amino groups, have the property of “going on to” the Lewis-acidic fibers. Such silicone amine oils and also their uses are described in EP 1555011 A for example.
Both mechanisms produce a permanence which is only transient and insufficient, allowing the coating to be easily removed not only mechanically but also chemically.
Appreciably better permanences are achieved when the fixing of polymer and substrate or crosslinking of polymer is effected via the formation of covalent bonds.
Covalent crosslinking can be effected for example by crosslinking the silicone polymers even as they are being prepared, by using trifunctional building blocks for example. However, the polymers thus obtained are thereby adversely affected in their processing properties (for example, melt viscosities, formability, solubility in an application auxiliary). Nor is any fixing to the substrate generally possible any longer. Therefore, subsequent fixing/crosslinking following the performance of an application step is always more sensible.
Such subsequent fixing/crosslinking can be effected for example by the presence of alkoxysilyl groups in the silicone polymer which ensure better permanence through hydrolysis and condensation with hydroxy groups of the substrate or hydroxy groups of other silicone polymers. Such alkoxysilyl-containing silicone polymers are described in EP 1544223 A1 for example. However, the Si—O—C or Si—O-E (E=element of substrate) bonds which form on attachment to the substrate are generally hydrolysis-labile and therefore easy to open again, and therefore permanence in the aqueous environment in particular is generally not good. On the other hand, the formation of comparatively stable siloxane bonds Si—O—Si generally requires a prior treatment of the substrate with appropriate silanes.
Another way of ensuring subsequent covalent crosslinking is to introduce (meth)acrylate groups into the silicone polymer. These groups are known to crosslink and cure on irradiation with UV light. Such photocurable silicone polymers are known and described in U.S. Pat. No. 5,635,544 for example. However, the reaction of methacrylate groups generally ensures only crosslinking between the individual polymer chains and not any fixation to the substrate. Methacrylic functionalization of the substrate, necessary for effective fixing, is costly and inconvenient, however.
N-Methylol crosslinking, already known in the area of the purely organic polymers, is another crosslinking mechanism. It involves the production of polymers bearing N-methylolamide groups by copolymerization with suitable monomers. These N-methylolamide groups are known to bond covalently to alcoholic groups in the absence of water at elevated temperature or, in the presence of acidic catalysts, at lower temperatures. They are similarly capable of reacting with each other and of so effecting a crosslinking of the polymer. Both cases give rise to covalent ether bonds which are known to be very strong and to break only under extreme physical or chemical loads. This effect is utilized for example by EP 0 143 175 A, which uses a free-radical emulsion polymerization to produce polymer dispersions which are postcrosslinkable via the methylol mechanism discussed above. Methylolamide groups can in principle be prepared by reaction of amines with formaldehyde, but the reaction leads in general to polymeric condensation products which via imine intermediates leads to polymeric networks. This reaction of amines with formaldehyde has already been described:
U.S. Pat. No. 3,461,100 describes condensation products of aldehydes and primary di- and monoamines. The resulting highly polymeric condensation products are discussed as protective coatings. DE 10047643 A1 describes polymeric condensation products of aldehydes and silicone amines, but which are exclusively present in highly polymeric and highly crosslinked form.
The product is already highly polymeric in the as-reacted state in both references. The product is accordingly no longer present in a reactive form, such as that represented by the monoaddition product of a formaldehyde molecule onto an amine, and hence is also no longer available for descendent reactions onto substrates or postcrosslinking reactions between product molecules.